Exemplary inverter-driven load devices may include an electric motor, an uninterruptible power supply (UPS), an electromagnetic cooker, illumination and the like. All of these devices may suffer from insulation deterioration due to aging deterioration. For example, in case of an electric motor used for a transport machine, movement of a platform coupled to the motor may cause friction, twist or extension and contraction in a power feeding conductor cable, resulting in a damaged conductor film. In another case of an electric motor used for a cutting machine, a cutting fluid, oil or the like may be scattered onto the motor, and then run through a shaft thereof to encroach insulating materials inside the motor.
Thus, a degree of insulation deterioration of the inverter-driven load device can be varied depending on an environment in use or durability of members. Once a leakage current flows through a portion where this insulation deterioration occurs, causing a risk of electrifying human bodies or working an earth leakage breaker. The earth leakage breaker is installed for preventing the electrification of the human body. As a matter of course, the first priority is given to human life. However, once the earth leakage breaker is operated, apparatuses or facilities including the load device of interest are forced to be stopped. Thus, it will take a lot of time to identify how and where the electric leakage has been caused as well as to restore the apparatuses or facilities, thereby lowering the operation efficiency thereof.
Exemplary means for measuring such a leakage current is disclosed in FIG. 2 of Patent Document 1, FIG. 1 of Patent Document 2, and FIG. 1 of Patent Document 3. On the feeding electric path to the inverter-driven load device, a zero-phase current transformer (ZCT) that measures the differential current component between the to and fro paths and the zero-phase current of the three-phase alternating current is arranged, or a current detector such as a current transformer (CT), a Hall CT, a shunt resistor or the like is arranged for each phase. Then, the leakage current is calculated by summing the output signals of all the phases.
In recent years, for example, as shown in FIG. 1 of Patent Document 4, proposed is an insulation deterioration diagnostic apparatus for detecting insulation deterioration in the following manner: a power supply circuit to an electric motor is switched using a switch to a closed circuit including an insulation resistance and the ground; an AC (alternating current) voltage applied to the control circuit of the electric motor is rectified by a rectifier circuit; and the obtained DC (direct current) voltage is used to measure a current flowing into the closed circuit. In this case, since the leakage current is a DC component, the zero-phase current transformer or the current transformer designed to detect an AC component cannot be used. Accordingly, any element that can detect a DC component, such as the Hall CT or the shunt resistor, is used.